Low-cycle Fatigue Behavior Of Incoloy 825 Nickel-based Superalloy

Because of their excellent high temperature fatigue resistance and oxidation resistance as well as stress-corrosion resistance,the nickel-based superalloys have been widely used in such industrial fields as the mechanical equipment,aviation and petrochemical industries.These alloys are used to make the key engineering components such as industrial gas turbine blades which are operating under the uncharitable conditions.It is thought that the fatigue is one of main failure forms for key nickel-based superalloy engineering components.Thus,in this dissertation,through performing the total-strain controlled low-cycle fatigue tests at room temperature,650 oC and 760 oC for the Incoloy 825 nickel-based superalloy and its welding joint,the fatigue behaviors of both alloy and welding joint were investigated.The cyclic stress response behaviors,low-cycle fatigue life behaviors and cyclic stress-strain behaviors were determined,and the strain fatigue parameters at different temperatures were given.The dislocation substructures and fracture surface morphologies after the low-cycle fatigue deformation for both alloy and welding joint were respectively analyzed with the transmission electron microscope(TEM)and scanning electron microscope(SEM),in order to provide a reliable theoretical basis for the fatigue-resistant design of Incoloy 825 nickel-based superalloy engineering components.The results of low-cycle fatigue tests show that at room temperature,the Incoloy 825 nickel-based superalloy exhibits the cyclic strain hardening in the early stage of low-cycle fatigue deformation,while the cyclic strain softening is noted in the late stage of low-cycle fatigue deformation.At 650?C,the Incoloy 825 nickel-based superalloy exhibits the cyclic strain hardening in the early stage of low-cycle fatigue deformation,while exhibits the stable cyclic stress response or cyclic strain hardening in the late stage of low-cycle fatigue deformation.At 760?C,the Incoloy 825 nickel-based superalloy exhibits the cyclic strain hardening in the early stage of low-cycle fatigue deformation,while the cyclic strain softening or stable cyclic stress response can be noted in the late stage of low-cycle fatigue deformation.At room temperature,the Incoloy 825 nickel-based superalloy welding joint exhibits the stable cyclic stress response,cyclic strain hardening or cyclic strain softening in the early stage of low-cycle fatigue deformation,while exhibits the cyclic strain softening in the late stage of low-cycle fatigue deformation.At 760?C,the Incoloy 825 nickel-based superalloy welding joint exhibits the cyclic strain hardening in the early stage of low-cycle fatigue deformation,while the cyclic strain softening is noted in the late stage of low-cycle fatigue deformation.At the same total strain amplitude,the fatigue lives of Incoloy 825 nickel-based superalloy are the longest at room temperature,and are the shortest at 760?C.At the same total strain amplitude,the fatigue lives of Incoloy 825 nickel-based superalloy welding joint at room are longer than those at 760?C.The fatigue lives of Incoloy 825 nickel-based superalloy are longer than those of Incoloy 825 nickel-based superalloy welding joint at room temperature and 760?C.The relationship between both plastic strain amplitudes and elastic strain amplitudes as well as reversals to failure at different temperatures for the Incoloy 825 nickel-based superalloy and welding joint shows a single slope linear behavior,which can be described by the Coffin-Manson and Basquin equations,respectively.In addition,the cyclic stress-strain relation at different temperatures for the Incoloy 825 nickel-based superalloy and welding joint shows also a single slope linear behavior.The observation results on the dislocation substructures after the low-cycle fatigue deformation at different temperatures and total strain amplitude for the Incoloy 825 nickel-based superalloy with transmission electron microscope reveal that the low-cycle fatigue deformation mechanism is mainly the planar slip.The sub-grains,dislocation array and cellular substructures can form during the low-cycle fatigue deformation.The observation results on the morphologies of low-cycle fatigue fracture surfaces for the Incoloy 825 nickel-based superalloy under different conditions with scanning electron microscope show that the fatigue cracks initiate and propagate in the transgranular mode.